fix RTD build #1616

Job	Run time
test-getting_started / tests	1m 59s
test-tutorials (configuration.ipynb) / tests	1m 34s
test-tutorials (debugging.ipynb) / tests	2m 24s
test-tutorials (describing_tasks.ipynb) / tests	2m 37s
test-tutorials (multiple_pilots.ipynb) / tests	2m 45s
test-tutorials (profiling.ipynb) / tests	2m 9s
test-tutorials (raptor.ipynb) / tests	2m 8s
test-tutorials (raptor_mpi.ipynb) / tests	5m 19s
test-tutorials (submission.ipynb) / tests	1m 26s
test-tutorials (staging_data.ipynb) / tests	2m 9s
	24m 30s

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